U.S. Pat. No. 6,566,986 discloses a dielectric filter composed of three or more rectangular parallel-piped dielectrics connected in line or integrally formed on a dielectric block. FIG. 7 illustrates one such waveguide-type dielectric filter. The filter comprises a metal coated rectangular parallel-pipe shaped dielectric block with input/output contact pads 112 made of a conducting film 71 as island-type contact pads surrounded by the insulating material of the dielectric block, and through holes 72 interposed between adjacent resonators 114 to form the resonators and coupling irises. As one skilled in the art would understand, the size and location of the contact pads 112, together with the size of the coupling irises, determine the bandwidth of the filter. This type of conventional filter does not provide a substantially wide bandwidth which is required for many applications, such as microwave communications, Radar, or other applications for which devices having frequency selective transmission characteristics are desired.
In order to obtain a wideband filter, the coupling of the filter to the external circuitry needs to be increased, which has typically been achieved by using a larger contact pad. However, since the size of the contact pad must remain smaller than the broad face of the resonator itself on which the pad is formed, it is still not possible to provide wide-bandwidth designs with this kind of structure. Another method of increasing the coupling involves reducing the height of the dielectric block in the thickness direction. Ultimately, however, this technique is likewise not practically useful, because the reduced thickness of the dielectric block needed to provide the increased coupling also reduces the resonator Q, which, in turn, undesirably increases filter insertion loss.
The spurious band rejection is also serious issue related to waveguide-type filters. The resonators of the filters have high order resonance modes occurring at high frequencies which result in spurious (and normally undesired) pass-bands. These higher order mode resonances introduce undesired transmission at these frequencies, and also limit the ultimate rejection of the filter.
In view of the above, it would be desirable to provide a waveguide filter that can provide a wide pass-band without negatively affecting the resonator Q, and which has an improved spurious response.